arc: apply arc_min tunables set before arc_init() has run

Tunables are applied as each parameter is registered, and zfs_arc_min is
registered before zfs_arc_max.  So param_set_arc_min() can run while
arc_c_max is still zero, and its upper bound check rejects every
non-zero value:

    Setting sysctl vfs.zfs.arc.min failed: 22

The value is dropped and the ARC floor falls back to allmem/32.
param_set_arc_max() is not affected, because it tests a lower bound
against arc_c_min, which a zero value satisfies.

Skip the upper bound while arc_c_max is zero and store the raw value.
arc_init() expects this: arc_set_limits() sets both limits, and the
arc_tuning_update() call after it applies zfs_arc_min.

Storing the value exposes a second bug.  The other arc handlers call
arc_tuning_update() unconditionally, so one registered before arc_init()
reaches WARN_IF_TUNING_IGNORED with both limits still zero and reports
the tunable as ignored, even though arc_init() applies it soon after:

    WARNING: ignoring tunable zfs_arc_min (using 0 instead)

Return early from arc_tuning_update() while arc_c_max is zero, which
covers every caller.  Also guard the writeback, so the tunable is not
overwritten with a zero limit.

This is the warning reported in #12504.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Abdelkader Boudih <[email protected]>
Closes #12504
Closes #19008

module/os/freebsd/zfs/sysctl_os.c

@@ -198,7 +198,7 @@ param_set_arc_max(SYSCTL_HANDLER_ARGS)
 	arc_tuning_update(B_TRUE);
 
 	/* Update the sysctl to the tuned value */
-	if (val != 0)
+	if (val != 0 && arc_c_max != 0)
 		zfs_arc_max = arc_c_max;
 
 	return (0);
@@ -215,14 +215,15 @@ param_set_arc_min(SYSCTL_HANDLER_ARGS)
 	if (err != 0 || req->newptr == NULL)
 		return (SET_ERROR(err));
 
-	if (val != 0 && (val < 2ULL << SPA_MAXBLOCKSHIFT || val > arc_c_max))
+	if (val != 0 && (val < 2ULL << SPA_MAXBLOCKSHIFT ||
+	    (arc_c_max != 0 && val > arc_c_max)))
 		return (SET_ERROR(EINVAL));
 
 	zfs_arc_min = val;
 	arc_tuning_update(B_TRUE);
 
 	/* Update the sysctl to the tuned value */
-	if (val != 0)
+	if (val != 0 && arc_c_max != 0)
 		zfs_arc_min = arc_c_min;
 
 	return (0);

module/zfs/arc.c

@@ -7624,6 +7624,9 @@ arc_tuning_update(boolean_t verbose)
 {
 	uint64_t allmem = arc_all_memory();
 
+	if (arc_c_max == 0)
+		return;
+
 	/* Valid range: 32M - <arc_c_max> */
 	if ((zfs_arc_min) && (zfs_arc_min != arc_c_min) &&
 	    (zfs_arc_min >= 2ULL << SPA_MAXBLOCKSHIFT) &&